This invention relates to dynamoelectric machines such as motors and alternators and more particularly to an exciter assembly for use in brushless dynamoelectric machines.
Motors and alternators, generically referred to as dynamoelectric machines, have in common the need to supply an excitation current to a rotating coil to produce a rotating magnetic field. This current may be supplied to the rotating coils through a set of slipping contacts, known as brushes, or through a brushless connection wherein the exciter current source is mounted directly on the rotating shaft on which the rotating coils of the dynamoelectric machine are also mounted.
The use of brushes and rings to transmit the exciter current is commonly and widely used, but suffers from the frequent need to replace the brushes that wear out due to the continuing rubbing against the contact rings on the rotating shaft.
The prior art has attempted to resolve this problem with structures that eliminate brushes and rings. In such structures the exciter itself is mounted onto the rotating shaft and is directly connected to coils that are also mounted on the shaft, thus enabling rotation of the rotor and the coil in synchronization without requiring slipping contacts such as rings and brushes.
There are a vast number of patents attempting to provide a satisfactory shaft-mounted exciter. Typical such arrangements are shown, for example, in U.S. Pat. No. 3,401,328, issued to Hartung; U.S. Pat. No. 4,210,857, issued to Korbell; and U.S. Pat. No. 4,307,309, issued to Barrett, and U.S. Pat. No. 5,705,872, issued to Loge.
A common design of brushless dynamoelectric machines, shown in FIG. 1, incorporates a primary alternator 10 and a secondary alternator 12, each having rotors, 11 and 13 respectively, rotated on a crankshaft 14. In secondary alternator 12, current flowing in the winding 15 of stator 16 generates a magnetic field through which windings 17 on the secondary alternator rotor 13 pass. Current induced in windings 17 of secondary alternator rotor 13 is transmitted to field winding 18 on rotor 11 of primary generator 10, which then rotates to generate the main current in winding 19 of stator 20. Because windings 17 of secondary alternator rotor 12 are rotating with field winding 18 of primary generator 10, the need for brushes and slip rings is eliminated.
Mounting the exciter on the rotating shaft may create difficulties due to the weight of the exciter and the need to keep the rotating machine balanced. In many applications, the shaft rotates at high speeds in excess of 2000 rpm. To prevent vibration and eventual destruction of the shaft bearings, it is essential to develop exciters that have great rotational symmetry and that are lightweight. The exciter must be provided with external power. This is typically accomplished by inductively coupling the rotating portion of the exciter circuit to the stationary part. It is thus desirable to provide an exciter structure that has excellent energy transfer characteristics between the stationary external source and the rotating electronics on the rotor shaft in addition to being easy to manufacture, compact, lightweight, and able to be easily retrofitted to existing dynamoelectric machines.
The present invention achieves these desirable characteristics through a novel exciter for a dynamoelectric machine. The exciter comprises a shaft adapted to rotate about a first axis, a cylindrical ferrite core mounted to the shaft, and a ferrite casing adapted to remain stationary and having a cylindrical inner surface coaxial with the first axis. A first transformer winding comprises a first conductor wound circumferentially about the ferrite core co-axially with the first axis and having a first diameter. A second transformer winding comprises a second conductor mounted on the ferrite casing inner surface concentric with the first transformer winding and having a second diameter larger than the first diameter by an amount just sufficient for the first transformer winding to rotate without contacting the second transformer winding. The ferrite core and the ferrite casing form a substantially complete enclosure for the first transformer winding and the second transformer winding. The second transformer winding may be self-supporting.
The exciter may further comprise a multivibrator mounted on the ferrite casing and electrically connected to the second transformer winding and to a voltage regulator. A plurality of diode rectifiers may be symmetrically balanced about the shaft, electrically connected to the rotating coil, and configured as a full-wave bridge diode rectifier having an output. A rotor may be attached to the shaft and may have a rotor winding connected to the output of the diode rectifier, and a stator having a stator winding may surround the rotor and be electrically connected to a bridge rectifier having an output. The voltage regulator may be electrically connected to the output of the bridge rectifier and may comprise a switching circuit adapted to interrupt power to the multivibrator when a voltage from the bridge rectifier exceeds a first predetermined value and to restore power to the multivibrator when the voltage drops below a second predetermined value. The multivibrator may further be connected to an external source of power via a switch. The external source of power may be a battery to which the bridge rectifier output is also connected.